Web Animations CSS: Dont decompose when interpolating with the empty transform list

r162457 [1] attempted to force transform animations to be animated on the
comositor (as in the legacy engine) by substituting an empty transform list
with the identity matrix. This had the side effect of forcing interpolation
with an empty list down the matrix decomposition path resulting in
interpolation errors.

This patch avoids these problems by converting only the output of transform
interpolation from the empty transform list to the identity matrix.

[1] http://src.chromium.org/viewvc/blink?view=revision&revision=162457

BUG=326952

Review URL: https://codereview.chromium.org/109993003

git-svn-id: svn://svn.chromium.org/blink/trunk@163489 bbb929c8-8fbe-4397-9dbb-9b2b20218538